Thursday, May 21, 2015

Tiny Blinky user guide

If you were at MakerFair Bay Area 2015, you may have seen (or hopefully even purchased) a pair of little round blinking earrings from us.

I've finally gotten around to fulfilling the "open" part of "open hardware" - this page.

The circuit is simplicity itself: An Attiny84 with 8 LEDs, an SMD pushbutton and a CR1220 battery clip. The only other component is a 10k pull-up resistor on !RESET. The series resistors for the LEDs are present, but it turns out that 0 ohms is the best value to use.

Normally, it's always a good idea to add a 0.1 µF bypass cap close to the Vcc pin of any logic component. In this particular case, however, we can get away with omitting it because the ATTiny is the only load, and the battery is so physically close.

If you push the button briefly (<250 ms), it will change the pattern (remembering the selected one in EEPROM). If you push the button longer (>250 ms), it will power the blinky off. While it's powered off, the battery will last "indefinitely" (limited only by its own shelf life). While it's running, you can expect it to last for an evening or so (we had better luck at Maker Faire - they lasted the whole weekend, albeit powered off overnight).

There isn't a lot to say about how it works. The one thing of note is that the ISP programming pins should not be used for LEDs, since there are no series resistors. Programming could possibly wind up pushing too much current through them. MOSI and SCK are therefore connected only to the ISP header, and MISO is used only for the button (so don't push it while programming is in progress).